The invention relates to a system for identifying a called terminal amongst a plurality of user terminals connected to a telephone switch which is connected to a conventional telephone network by at least one fixed radiotelephone terminal serving to replace a cable link. Each fixed radiotelephone terminal performs functions analogous to those of a conventional portable ratio-telephone terminal, however it serves to connect the telephone network to the switch or exchange rather than to a single user.
By way of example, the switch is a small switch serving a hotel or a village situated in an isolated region where it would be too expensive to lay a cable. The switch serves a group of fixed user terminals by routing calls coming from the public network. Each fixed radiotelephone terminal is used in succession with different user terminals since the user terminals are not in permanent use. A plurality of radiotelephone terminals can share a common frequency band by using time division or code division.
The invention relates more particularly to radiotelephone networks of the GSM type, and to those of types derived from the GSM type and which include a stationary satellite, or satellites in low earth orbit (LEO). GSM type networks have the following characteristics:
a geographical zone subdivided into cells so as to enable carrier frequencies to be reused;
a gateway that manages the resources of each cell concerning carrier frequencies and a plurality of channels carried by each carrier frequency; and
at least one switching center of the mobile radio service that manages: calls, connections with the fixed telephone network, and databases, wherein the databases contain the directory number, the international mobile subscriber number, and the location of each mobile subscriber.
Conventional GSM networks or networks derived therefrom are not designed to enable a plurality of subscribers to use the same user terminal. The user personalizes the terminal being used by inserting a subscriber identification module (SIM) card therein containing the international mobile subscriber identity (IMSI) which determines the account to which calls will be billed. After an initialization stage, the terminal is specified in radio messages by a temporary mobile subscriber identity (TMSI) which serves to protect the user""s anonymity. In addition, an authentication procedure uses a secret key contained in the SIM card to prevent a user making fraudulent use of the IMSI or the TMSI of another user, whenever a call is made or received.
The use of a plurality of fixed radiotelephone terminals to connect a single switch to a GSM network raises the problem of supplying the switch with the identity of the called line.
The object of the invention is to propose a simple system for solving this problem.
The invention provides a system for identifying a called terminal amongst a plurality of user terminals connected to a telephone switch, said telephone switch being connected to at least one fixed radiotelephone terminal, each fixed radiotelephone terminal including means for communicating wit a fixed telephone network; the system including call forwarding means for forwarding a call addressed to one of the user terminals and containing the directory number of the called user terminal to a mobile service switching center giving access to the called user terminal, wherein:
the call forwarding means for forwarding a call addressed to one of the user terminals includes means for sending to the mobile service switching center an IAM message containing the directory number of the called user terminal;
wherein the mobile service switching center further includes means for supplying the directory number to a fixed radiotelephone terminal in a SETUP message; and
wherein each of said fixed radiotelephone terminals includes means for extracting the directory number of the called user terminal from the SETUP message received for setting up each call, and for sending said number to the switch in the form of a conventional signalling message.
The system characterized in this way makes it possible in simple manner to perform direct inward dialing, and the switch can behave like a switch connected to a fixed network in order to perform direct inward dialing.